A global system for mobile communication (GSM) is a radio technology which has been developed as a system for standardizing radio communication systems in Europe and which has widely been deployed all over the world. A general packet radio service (GPRS) is introduced to provide a packet switched data service in a circuit switched data service provided from the GSM.
An enhanced data rate for GSM evolution (EDGE) employs 8-phase shift keying (PSK) as well as Gaussian minimum shift keying (GMSK) employed in the GSM. An enhanced general packet radio service (EGPRS) represents the GPRS using the EDGE. An enhanced general packet radio service phase 2 (EGPRS2) supporting further various modulation and coding schemes has recently being developed. While the EGPRS supports only two modulation schemes (i.e., GMSK and 8-PSK), the EGPRS2 supports five modulation schemes (i.e., GMSK, quadrature phase shift keying (QPSK), 8-PSK, 16-quadrature amplitude modulation (QAM), and 32-QAM). There are two levels of the EGPRS2, that is, EGPRS2-A and EGPRS2-B. Up to now, the EGPRS2-A supports the GMSK, 8-PSK, 16-QAM, and 32-QAM, and the EGPRS2-B supports the GMSK, QPSK, 16-QAM, and 32-QAM. To diversify the modulation and coding scheme of the EGPRS2, there is an ongoing standardization task in which the 8-PSK is added to a downlink EGPRS2-B so that the five modulation schemes (e.g., GMSK, QPSK, 8-PSK, 16-QAM, and 32-QAM) are supported when using the EGPRS2-B. Hereinafter, an EGPRS system denotes a system supporting the EGPRS or a system supporting the EGPRS and the EGPRS2.
The EGPRS system provides a multi-data rate by using various modulation and coding schemes. The data rate is regulated on the basis of channel quality in a link adaptation process. If the channel quality is good, data is transmitted with a high data rate. On the contrary, if the channel quality is poor, data is transmitted with a low data rate. Data may be lost when the data is transmitted according to a modulation and coding scheme that requires a data rate higher than that achievable with the channel quality. In link adaptation, a data throughput is increased using a highest possible data rate by using a specific modulation and coding scheme with a predetermined channel quality.
For the link adaptation process, the channel quality needs to be reported from a mobile station (MS) to a base station (BS). In the EGPRS system, the channel quality between the BS and the MS is represented with a bit error probability (BEP). The BEP is an expectation value of an actual bit error rate (BER) of a signal received by the MS through a radio channel. The BEP is measured on a burst-by-burst basis. The BS selects a suitable modulation and coding scheme according to the reported channel quality. The link adaptation can be performed most efficiently when the actual BER is most accurately estimated by the reported BEP.
A reduced transmission time interval (RTTI) configuration is one of techniques introduced in the EGPRS system. Unlike a basic transmission time interval (BTTI) configuration in which one packet data channel (PDCH) is used to configure a radio block consisting of four bursts, in the RTTI configuration, two PDCHs (also referred to as a PDCH-pair) are used to configure the radio block consisting of four bursts. Therefore, a time for transmitting the radio block in the RTTI configuration is only half of that of the BTTI configuration.
The channel quality is required so that a network can know a channel condition and determine an optimal modulation and coding scheme. In an arbitrarily varying channel environment, the channel condition is considered as accurately as possible during a time when the channel quality is reported. However, which scheme will be used in the RTTI configuration to report the channel quality has not been introduced unlike in the BTTI configuration.